1. Field of the Invention
The present invention pertains to the manufacture of capacitors of the type that are stacked and obtained by winding on a large-diameter wheel.
2. Description of the Prior Art
This winding method is now widely used. Metallized dielectric films are generally used. The winding is done so as to make a lateral offset between two successive foils of the stack so that electrical contacts can subsequently be deposited on the sides of the stack. Once the stack is made, it is subjected to heat treatment intended to stabilize its electrical properties, especially the dielectric loss factor and the capacitance value. This heat treatment also gets rid of air inclusions between the foils of the stack. The treatment comprises an operation to press the stack directly on the winding wheel at fairly high temperatures (about 240.degree. C.). The winding wheel is generally made of aluminum and its diameter is generally about 800 millimeters. The heat treatment causes the wheel to expand and consequently causes a major elongation of the mother capacitors during baking. Between 20.degree. C. and 240.degree. C., an aluminum wheel with a diameter of 800 mm. undergoes an elongation of 4 mm. in its diameter, corresponding to more than 12 mm. in its circumference.
The elongation that results from the mother capacitors counteracts the beneficial effects of the baking process itself. To remove this disadvantage, it is possible to use wheels made of materials with a zero expansion coefficient, for example, invar. These materials are, unfortunately, very expensive and are not used in this specific case because they excessively increase the cost price of the capacitors.
To remove these disadvantages, the invention proposes the use of a wheel of a special design which makes it posssible to associate different materials. The wheel according to the invention has several parts including, especially, a rim which is detachable from the wheel body and has a radial gap so that it can expand freely under the effect of heat. This rim, which is made of a material generally used in this type of technology (aluminium for example), is clamped by a hoop made of a material with an expansion coefficient which is zero or practically zero. The hoop therefore sets a constant external diameter for the wheel and enables the free expansion of the rim.
3. Summary of the Invention
An object of the invention is therefore a winding wheel for stacked capacitors, comprising a wheel body on which a rim is fixed detachably, the circumference of the said rim being cut along a certain length, the rim being clamped within a hoop made of a material with an expansion coefficient of zero, the said cut length being calculated so that, when the rim and the hoop which clamps it are disconnected from the wheel body and carried to a determined temperature, the ends of the rim defined by the cut do not touch one another.